1. Field Of The Invention
The present invention relates to a high resolution optical microscope and an irradiation spot beam-forming mask for the optical microscope, and more particularly to a high resolution optical microscope, wherein an object of a small size of 0.1 .mu.m or less can be observed by a phase shifting mask on which a phase arrangement, each of neighboring portions in the arrangement is optically phase-shifted from one another at 180.degree., is regularly arranged in the two-dimensional direction, and not only visible rays but infrared rays, ultraviolet rays and X-rays are applicable to the optical microscope as long as the used light beam is coherent.
2. Description Of The Prior Art
Conventionally, in a high resolution optical microscope, typical one of three methods is used: an interference method, a dark field method or a near-field scanning method.
In the optical microscope using the interference method, a resolution in the longitudinal direction, to which an optical axis is parallel, is relatively high, but a resolution in the lateral direction is at most on the order of about 1 .mu.m because a diameter of the irradiation spot beam can not be made small under a diameter of an Airy disk.
In the optical microscope using the dark field method, only an object to be measured (e.g., a contaminant or foreign matter) brightens and image contrast is high. However, it is necessary to completely prevent the illumination light from entering an objective lens, there is such a problem that magnification extremely differs with the size and the direction of the object to be measured.
In the near-field scanning optical microscope (NSOM) using the near-field scanning method, since luminous flux output from a minute pinhole of an optical probe scans an object in the two-dimensional direction within the near field of the object, resolution in the lateral direct-ion is high ("Development of a Fluorescent Method of Near-field Scanning Optical Microscope and Trial of Application to Bioobservation", by Satoshi Okazaki, O plus E, No. 118, Sep. 1989, pp. 110-116) However, it is impossible for some objects to be scanned by the flux in the near field. There are many restrictions on the use because of the receiving of the faint light.
In contrast, a scanning tunneling microscope (STM) with a tunnel electric current has been developed,, and can be expected to measure an object in the unit of atoms. In this microscope, however, it is necessary for a metallic probe to approach a conductive sample as close as 1 nm and to scan a surface of the sample so as to maintain a generated tunnel current constant. As a result, a precision actuator and a sophisticated servo mechanism are required. In addition, the scanning tunneling microscope has such disadvantages that an insulating sample can not be measured, that a wide area (over several tens of micrometers) of a sample can not be consecutively measured, and that a measured absolute value is somewhat unreliable and the like.
The high resolution of 0.1 .mu.m or less can be also obtained by using a scanning electric microscope (SEM). Since, however, an electron beam scans an object in a vacuum, the apparatus is made large and complicated. In addition, the microscope has such a disadvantage that there is a restriction on materials of an object to be measured, for example, a living body can not be measured, and a sophisticated scanning servo mechanism is required because of a single electron beam.
As described above, the resolution or the like is limited in the conventional optical microscopes using the interference method or the dark field method. In the conventional scanning microscope such as the optical microscope using the near-field scanning method, there are such problems that a complicated and expensive servo mechanism or the like is required, and that it is impossible for the scanning microscope to observe a two-dimensional plane surface or a three-dimensional solid surface of a sample without scanning.